This invention relates to voltage controlled oscillators, and more specifically to voltage controlled oscillators which exponentially increase the frequency of the output signal with a linearly increasing input voltage signal.
A voltage controlled oscillator produces an oscillating waveform which varies in frequency in some relationship with an input voltage signal. Voltage controlled oscillators with an exponential transfer characteristic have several applications. Among these applications are use as a test oscillator for recording the acoustic characteristics of a room, a test oscillator for recording the acoustic characteristics of audio components, a test oscillator used in physical hearing examinations, and as an oscillating waveform generator. One of the more important applications of voltage controlled oscillators with an exponential transfer characteristic is in the generation of electronically synthesized music. An example of an oscillator intended for use in the electronic synthesis of music is shown in Rocheleau, "Voltage-Controlled Exponential Oscillator", U.S. Pat. No. 3,605,035, which is hereby incorporated by reference.
A common method of generating the exponential transfer characteristic is to bias a diode in its subthreshold operating range. The use of this technique is described in Robert A. Moog, "Voltage-Controlled Electronic Music Modules", Journal of the Audio Engineering Society, July 1965, which is hereby incorporated by reference. However, when the voltage controlled oscillator is to be incorporated in an integrated circuit including complimentary metal oxide semiconductor devices, it is difficult to construct a diode with the accurate voltage versus current characteristic necessary to utilize the subthreshold biasing technique.